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Earlier Water on Earth? Oldest Rock Suggests Hospitable
Young Planet

Cathodoluminescence image of the oldest
known material from the Earth, a single
crystal of zircon from the Jack Hills
metaconglomerate, Western Australia. Concentric,
magmatic growth zoning is shown about
the crystal core. The crystallization
age of 4.40Ga (4004+-4Ma) was determined
from the circled area by ion microprobe.
The arrow points to an inclusion of quartz.
The high oxygen isotope ratio from this
sample suggests that low temperature surficial
processes including liquid water were
important for the formation of protoliths
to this magma."

Geological evidence suggests that Earth may have had
surface water --and thus conditions to support life
-- millions of years earlier than previously thought.
Scientists reconstructed the portrait of early Earth
by reading the telltale chemical composition of the
oldest known terrestrial rock. The 4.4-billion-year-old
mineral sample suggests that early Earth was not a
roiling ocean of magma, but instead was cool enough
for water, continents, and conditions that could have
supported life. The age of the sample may also undermine
accepted current views on how and when the moon was
formed. The research was supported in part by the
National Science Foundation (NSF), and is published
in this week's issue of the journal Nature.

"This appears to be evidence of the earliest existence
of liquid water on our planet," says Margaret Leinen,
assistant director of NSF for geosciences. "If water
occurred this early in the evolution of earth, it
is possible that primitive life, too, occurred at
this time."

By probing a tiny grain of zircon, a mineral commonly
used to determine the geological age of rocks, scientists
from the University of Wisconsin-Madison, Colgate
University, Curtin University in Australia and the
University of Edinburgh in Scotland have found evidence
that 4.4 billion years ago, temperatures had cooled
to the 100-degree Centigrade range, a discovery that
suggests an early Earth far different from the one
previously imagined.

"This is an astounding thing to find for 4.4 billion
years ago," says John Valley, a geologist at UW-Madison.
"At that time, the Earth's surface should have been
a magma ocean. Conventional wisdom would not have
predicted a low-temperature environment. These results
may indicate that the Earth cooled faster than anyone
thought." Previously, the oldest evidence for liquid
water on Earth, a precondition and catalyst for life,
was from a rock estimated to be 3.8 billion years
old.

The accepted view on an infant Earth is that shortly
after it first formed 4.5 to 4.6 billion years ago,
the planet became little more than a swirling ball
of molten metal and rock. Scientists believed it took
a long time, perhaps 700 million years, for the Earth
to cool to the point that oceans could condense from
a thick, Venus-like atmosphere. For 500 million to
600 million years after the Earth was formed, the
young planet was pummeled by intense meteorite bombardment.
About 4.45 billion years ago, a Mars-size object is
believed to have slammed into the Earth, creating
the moon by blasting pieces of the infant planet into
space.

The new picture of the earliest Earth is based on a
single, tiny grain of zircon from western Australia
found and dated by Simon Wilde, of the School of Applied
Geology at Curtin University of Technology in Perth,
Western Australia. Valley worked with William Peck,
a geologist at Colgate University, to analyze oxygen
isotope ratios, measure rare earth elements, and determine
element composition in a grain of zircon that measured
little more than the diameter of two human hairs.
Colin Graham's laboratory analyzed the zircon to obtain
the oxygen isotope ratios. Graham is a contributor
to the paper and geochemist at the University of Edinburgh.

"What the oxygen isotopes and rare earth analysis show
us is a high oxygen isotope ratio that is not common
in other such minerals from the first half of the
Earth's history," Peck says. In other words, the chemistry
of the mineral and the rock in which it developed
could only have formed from material in a low-temperature
environment at Earth's surface.

"This is the first evidence of crust as old as 4.4
billion years, and indicates the development of continental-type
crust during intense meteorite bombardment of the
early Earth," Valley says. "It is possible that the
water-rock interaction (as represented in the ancient
zircon sample) could have occurred during this bombardment,
but between cataclysmic events."

Scientists have been searching diligently to find samples
of the Earth's oldest rocks. Valley and Peck say such
ancient samples are extremely rare because rock is
constantly recycled or sinks to the hot mantle of
the Earth. Over the great spans of geologic time,
there is little surface material that has not been
recycled and reprocessed in this way.

The tiny grain of zirconium silicate or zircon found
by Wilde in western Australia was embedded in a larger
sample containing fragments of material from many
different rocks, Valley says. Zircons dated at 4.3
billion years were reported from the same site a decade
ago, but the new-found zircon grain is more than 100
million years older than any other known sample, giving
scientists a rare window to the earliest period of
the Earth.

"This early age restricts theories for the formation
of the moon," Valley says. "Perhaps the moon formed
earlier than we thought, or by a different process."
Another intriguing question is whether or not life
may have arisen at that early time. Low temperatures
and water are preconditions for life. The earliest
known biochemical evidence for life and for a hydrosphere
is estimated at 3.85 billion years ago, and the oldest
microfossils are 3.5 billion years old.

"It may have been that life evolved and was completely
extinguished several times" in catastrophic, meteorite-triggered
extinction events well before that, Valley says. The
research conducted by Valley, Peck, Graham and Wilde
was also supported by the U.S. Department of Energy,
the U.K. Natural Environment Research Council and
a Dean Morgridge Wisconsin Distinguished Graduate
Fellowship.